Passive solar technologies are means of using sunlight for useful energy without use of active mechanical systems (as contrasted to active solar). Such technologies convert sunlight into usable heat (water, air, thermal mass), cause air-movement for ventilating, or store heat for future use, with little use of other energy sources. A common example is a solarium on the equator-side of a building. Passive cooling is the use of the same design principles to reduce summer cooling requirements.
Technologies that use a significant amount of conventional energy to power pumps or fans are active solar technologies. Some passive systems use a small amount of conventional energy to control dampers, shutters, night insulation, and other devices that enhance solar energy collection, storage, use, and reduce undesirable heat transfer.
Passive solar technologies include direct and indirect solar gain for space heating, solar water heating systems based on the thermosiphon, use of thermal mass and phase-change materials for slowing indoor air temperature swings, solar cookers, the solar chimney for enhancing natural ventilation, and earth sheltering.
More widely, passive solar technologies include the solar furnace and solar forge, but these typically require some external energy for aligning their concentrating mirrors or receivers, and historically have not proven to be practical or cost effective for wide-spread use. ‘Low-grade’ energy needs, such as space and water heating, have proven, over time, to be better applications for passive use of solar energy.
Advantages and comparisons to active solar
Passive solar systems have little to no operating costs, often have low maintenance costs, and emit no greenhouse gases in operation. They do, however, need to be optimized to yield the best performance and economics. Energy conservation reduces the needed size of any renewable or conventional energy system, and greatly enhances the economics, so it must be performed first. Passive solar technologies often yield high solar savings fractions, especially for space heating; when combined with active solar technologies or photovoltaics (for example, to power pumps or fans), even higher conventional energy savings can be achieved.